Ultrasonic transducers of the generic type have been disclosed in numerous publications. In using such ultrasonic transducers in ultrasonic proximity switches and distance sensors, there is a need, inter alia, for a narrow sensing characteristic. Objects can be detected specifically thereby, while, as desired, lateral disturbing reflectors, for example, in cramped surroundings or in narrow passages, exert no influence on the functioning of the sensor. This requires ultrasonic transducers which exhibit a very narrow sound lobe characteristic and at the same time have very small minor lobe amplitudes.
German Patent No. 25 41 492 C2 describes the sharply directional ultrasonic transducer in saturn geometry which is most widespread in ultrasonic sensors in industrial use. It is assembled from a piezoceramic disk, an adapting layer disk and a concentric metal ring. The face of the adapting layer is large owing to a wide projection of the adapting layer disk beyond the diameter of the piezoceramic disk, as a result of which a very narrow aperture angle .PHI. of 5.degree. is achieved for the sound lobe. Owing to the large projection of the adapting layer disk, it is difficult in practice always exactly to maintain the favorable vibration amplitude distribution and phase angle over the entire face, the result being problematic lateral minor lobes.
An ultrasonic transducer having a piezoceramic disk and a silicone adapting layer has furthermore been disclosed (see U.S. Pat. No. 3,928,777). The adapting layer has a diameter which is only slightly larger than the piezoceramic disk, the sound lobe opening angle .PHI. consequently being quite large at 10-12.degree.. The minor lobes are so small, with an attenuation of more than -25 dB, that they no longer play a role in practical use. Because of its low chemical resistance, the adapting layer material of silicone cannot be used in all instances of application, and moreover has a strong temperature drift of the mechanical characteristics.
Published International Patent Application No. WO 94/05004 discloses an ultrasonic transducer having a lateral groove in the adapting layer disk. The adapting layer comprises a robust and resistant syntactic foam as in the case of the transducer using saturn geometry. This design has the same minor lobe freedom as the transducer having silicone. Because of its likewise small projection of the adapting layer disk, it also has an equally large sound lobe aperture angle .PHI. of 10-12.degree..